Sufficient engagement of CD4+ T cells (TCD4+) is critical for a positive outcome in several human infections, including influenza (flu). TCD4+ are activated by complexes of pathogen-derived peptides (epitopes) and major histocompatibility class II molecules (MHCII) that are generated within the antigen bearing cell and then transported to the cell surface where they can be engaged by T cell receptor. According to convention, peptide-MHCII complexes are formed following internalization of extracellular (exogenous) antigen, proteolysis within the endocytic compartment and loading onto nascent MHCII in a late endosomal compartment. This classical pathway has been deduced mainly through study of durable globular proteins. When viruses are utilized, additional antigen processing schemes become apparent. What is more, our recent work is showing that these alternatives are not inconsequential. Indeed, through mainly endogenous pathways, they drive the bulk of the anti-influenza TCD4+ response in a C57Bl/6 (B6) mouse model. Thus, alternative MHCII antigen processing merits far greater attention than it is currently receiving. Organized into three independent but highly integrated specific aims, the work proposed here will: 1) investigate the mechanistic bases by which epitopes are processed by different pathways, 2) explore the possibility that not just unconventional antigen processing but also unconventional antigen-presenting cells (APCs) drive the response to an influenza lung infection, and 3) determine whether the processing pathway utilized to generate an epitope is a major determinant of TCD4+ expansion and functionality and, hence, protective capacity. These studies, a key component of our larger effort to expand the landscape of MHCII antigen processing, could substantially impact the rational design of vaccines against many pathogens. Further, they could point to new approaches to cancer immunotherapy, and provide important insight into the genesis and treatment of autoimmune diseases.